Amplifying optical fibers (e.g., optical fibers doped with rare-earth elements) are commonly used in numerous optical applications.
For example, erbium doping is used in long-distance optical telecommunications systems for amplifying transmitted optical signals. Such optical fibers are used in erbium-doped fiber amplifiers (EDFAs) and have a central core made of a silica matrix that includes rare-earth-dopant elements (e.g., erbium) at concentrations on the order of 250 parts per million (ppm) to 1000 ppm (i.e., 0.025 weight percent to 0.1 weight percent). The rare-earth-dopant elements may be associated with complementary dopant elements to improve amplification. For example, aluminum may be used as a complementary dopant element to broaden the gain band for wavelength division multiplex (WDM) applications.
Conventionally, optical amplification in a rare-earth-doped optical fiber operates by injecting a pump signal into the optical fiber, which excites the rare-earth elements (e.g., Er3+ in an EDFA). When a light signal passes through this portion of optical fiber, it de-excites the rare-earth elements by stimulated emission, thereby producing a photon that is identical in all respects to the incident photon. The light signal is thus multiplied by two.
The performance of a rare-earth-doped optical fiber is generally expressed in terms of power conversion efficiency (PCE). As shown in Equation 1 (below), the power conversion efficiency is the ratio of the gain of the amplifying optical fiber to the pump power used in order to obtain the gain. The gain of the amplifying optical fiber is defined by Equation 2.
                    PCE        =                                            P              out              S                        -                          P              in              S                                            P            in            P                                              Equation        ⁢                                  ⁢        1            Gain=PoutS−PinS  Equation 2
In these equations, PinP is the input pump power, PinS is the input signal power, and PoutS is the output amplified signal power.
In certain applications, it is desired to obtain high output powers from the amplifying optical fiber.
One solution involves increasing the concentration of rare-earth dopants in the central core of the optical fiber to increase the amplification gain.
Nevertheless, when the concentration of rare-earth dopants in the central core of the optical fiber is high, pairs or even aggregates of rare-earth elements can form in the core matrix (e.g., of silica) of the central core, thereby leading to non-uniform doping. Such doping non-uniformities reduce the amplification efficiency of the optical fiber because of the simultaneous existence of mechanisms other than the mechanism that provides amplification. These other mechanisms are, for example, resonant energy transfer, stepwise upconversion, cooperative luminescence, cooperative energy transfer, and simultaneous photon absorption. These mechanisms compete with stimulated emission and reduce the efficiency of light amplification. Such aggregates of rare-earth elements also accentuate photonic degradations that can occur in the central core of the optical fiber at high power during propagation of light signals in the optical fiber and as a result of crystal defects present in the core matrix (e.g., of silica) of the central core.
Another solution involves increasing the power of the pump signal. Nevertheless, depending on the value of the numerical aperture of the optical fiber, the energy conversion efficiency may be degraded.
FIG. 1 plots the variation in power conversion efficiency (PCE) as a function of pump signal power. FIG. 1 plots curves acquired for numerical aperture values of between 0.14 and 0.30.
Numerical aperture is an optical-fiber parameter that can be approximated by the following equation:NA=√{square root over (nc2−ng2)}where nc is the refractive index of the central core of the optical fiber and ng is the refractive index of the cladding of the optical fiber.
FIG. 1 illustrates that power conversion efficiency varies as a function of pump power. In particular, for a high numerical aperture value, the power conversion efficiency maximum occurs at low pump power values. For example, with a numerical aperture of 0.30, the power conversion efficiency maximum lies at a pump signal of about 75 milliwatts (mW). In contrast, for a small numerical aperture value, the power conversion efficiency maximum lies at high pump-power values. For example, with a numerical aperture of 0.14, the maximum power conversion efficiency occurs with a pump power of about 500 milliwatts.
In particular, for numerical apertures of less than 0.18, power conversion efficiency at high pump power (e.g., 500 milliwatts) becomes greater than 0.50. Furthermore, the power conversion efficiency varies little over a pump power range of 350 milliwatts to 500 milliwatts, so it is possible to vary the pump power over this range without significantly modifying power conversion efficiency.
Thus, for a given value of numerical aperture, there exists a maximum for power conversion efficiency, and decreasing the numerical aperture shifts the power conversion efficiency maximum towards higher pump-power values.
The explanation of this phenomenon comes from the fact that as the pump power injected into the optical fiber increases, high power densities in the central core give rise to a non-linear effect known as excited state absorption (ESA). When excited state absorption occurs, two pump photons are absorbed by a single rare-earth element, thereby exciting the rare-earth element to higher energy levels (i.e., energy levels that are higher than the typical energy levels appropriate for amplification). By relaxing in a non-radiative manner from these higher energy levels, the rare-earth element does contribute to amplification, but it does so as a result of consuming two pump photons, instead of only one. This loss mechanism reduces yield and, therefore, reduces power conversion efficiency. In other words, in order to obtain a given level of gain, it becomes necessary to use higher pump powers when ESA is present. By decreasing numerical aperture, the mode field diameter (MFD) of the pump signal is increased, thereby reducing the power density of the pump signal in the central core. The reduction of the pump signal's power density reduces the amplitude of excited state absorption, thereby improving power conversion efficiency.
The gain shape of an amplifying optical fiber designates the value of its gain as a function of the wavelength of the incident signal. For example, erbium-doped optical fibers are used to provide amplification in optical transmission systems, particularly for deployment within systems operating within the C band wavelength range. Typically, the C band includes wavelengths between about 1525 nanometers and 1570 nanometers (e.g., between about 1530 nanometers and 1565 nanometers). An erbium-doped optical fiber conventionally exhibits a gain width of about 30 nanometers to 35 nanometers in the C band and a numerical aperture of 0.23.
For high power applications, it is desirable to reduce numerical aperture to avoid losing amplification efficiency, while conserving gain characteristics.
The publication “Novel erbium-doped fiber for high power applications,” Passive Components and Fiber-based Devices, B. S. Wang et al., Proceedings of the SPIE, Vol. 5623, pp. 411-417 (2005), which is hereby incorporated by reference in its entirety, discloses rare-earth-doped optical fibers at high power for WDM applications. The Wang publication suggests that, for these kinds of fibers, the design of the optical waveguide should be adapted to ensure good overlap between the mode field diameter and the rare-earth elements (i.e., these rare-earth elements experience light, whereas rare-earth elements outside of the mode field will not provide amplification). Additionally, the design of the doping composition (e.g., the dispersion of the rare-earth dopant elements or the chemical environment) should be adapted to determine the gain shape of the EDFA fiber. The erbium-doped optical fiber of the Wang publication is usable at a pump power of 600 milliwatts. Nevertheless, the erbium doping is accompanied by a strong concentration of an aluminum complementary dopant element (i.e., a concentration greater than 12 molar percent) to improve gain width. Unfortunately, aluminum also increases the refractive index difference of the central core with respect to the cladding and increases background losses. It is possible to counter these increases by inserting fluorine to reduce the refractive index difference. However, the extent to which fluorine can be inserted is limited because fluorine can modify the gain width, particularly in the C band. Thus, in order to preserve gain in the C band, the optical fiber described in the Wang publication possesses a numerical aperture of less than 0.176.
European Patent No. 1,152,502 and its counterpart U.S. Patent Publication No. 2002/0003937, each of which is hereby incorporated by reference in its entirety, describe an optical fiber doped with erbium as well as alumina to improve the rare-earth doping. The optical fiber also includes germanium that adapts the value of the refractive index difference between the central core and the cladding to obtain a numerical aperture in the range 0.11 to 0.21. Nevertheless, the intended application is the L band (i.e., 1565 nanometers to 1625 nanometers).
Other solutions involve introducing rare-earth dopants into an optical fiber's central core by incorporating nanoparticles that are doped with rare-earth elements via modified chemical vapor deposition (MCVD). For example, European Patent No. 1,347,545 (and its counterpart U.S. Pat. No. 7,031,590) and International Publication No. WO 2007/020362 (and its counterpart U.S. Patent Publication No. 2009/0116798), each of which is hereby incorporated by reference in its entirety, describe optical fibers that include nanoparticles in the optical fiber's central core. The nanoparticles described in these documents include a rare-earth-dopant element together with at least one element that improves the amplification of the signal, such as aluminum, lanthanum, antimony, bismuth, or some other element. European Patent No. 1,347,545 discloses a final gain shape that is the sum of all the gain shape contributions linked to the plurality of different nanoparticles. The fiber design, nanoparticle manufacturing, and nanoparticle composition are different than in the present invention. International Publication No. WO 2007/020362 fails to disclose any optical fiber particle concentrations and has a different fiber design and nanoparticle composition.
French application Ser. No. 08/06752, which is hereby incorporated by reference in its entirety, describes an optical fiber that is rare-earth-doped by nanoparticles and that enables high powers to be obtained. The characteristics of the nanoparticles and of the doping are selected to ensure high gain in the optical fiber. In particular, the concentration of rare-earth elements is high to improve the gain of the optical fiber. Nevertheless, French application Ser. No. 08/06752 fails to disclose an optical fiber that possesses low numerical aperture for high-power applications.
Therefore, a need exists for a rare-earth-doped amplifying optical fiber with a small numerical aperture for high-pump-power applications without a degraded gain shape.